1. Field of the Invention
This invention relates to control of pollution of air by toxic substances and is more particularly directed to control of pollution of air by volatilized isocyanate generated in the formation of polymer foam bunstock.
2. Description of the Prior Art
The production of both rigid and flexible cellular polymers, based on polyisocyanates, on a continuous basis in a form known as "bunstock" is well-recognized in the art. The polyisocyanate-based foams include polyurethane foams, which are obtained by reaction of polyisocyanates with polyols such as polyether and polyester polyols; polyisocyanurate foams in which the polyisocyanate is trimerized, usually in the presence of minor amounts of polyols, as a part of the foam forming operation, and, more recently, polycarbodiimide-polyisocyanurate copolymer foams in which the polyisocyanate is subjected to the simultaneous action of trimerizing catalysts and carbodiimide-forming catalysts while the foam formation takes place.
The forming of the isocyanate-based foams on a continuous basis is conventionally carried out by dispensing the polymer foam forming ingredients at the top of an inclined conveyor belt which is in slow, but constant, motion. The belt generally carries a paper liner on which the actual foam is deposited. The conveyor is generally provided with two vertical sides at least in the portion immediately below the point at which the foam mix is deposited. There is thus formed a generally U-shaped mold in which the foam mix is shaped as it rises. The two vertical sides are normally in the form of continuous belts which move in synchronism with the inclined conveyor belt. Paper liners are interposed, by feeding in a continuous manner, between the edges of the rising foam and the vertical belts of the side walls. The above type of procedure is described in, for example, U.S. Pat. No. 3,152,361. In some types of operation the paper liners used to separate the side surfaces of the foam from the mold are caused to rise, at least in their initial path, in a direction parallel to the surface of the rising foam; see, for example, U.S. Pat. Nos. 3,091,811 and 3,719,734.
During the initial stages of the formation of the foam bunstock considerable heat is evolved due to the exothermic nature of the polymer foam forming reaction. For example, temperatures as high as 340.degree. F can be reached in the interior of polyurethane foams which are prepared by commercial bunstock operations. Such exotherms can cause volatilization of the polyisocyanate employed in the formation of the foam and give rise to potentially harmful pollution of the atmosphere in the vicinity of the bunstock machine. This problem is particularly severe in the case of toluene diisocyanate which, in the form of the 2,4- or 2,6-isomer or mixtures thereof, is the diisocyanate most commonly employed in the preparation of flexible polyurethane foams. Toluene diisocyanate is relatively volatile and significant pollution of the surrounding atmosphere can occur when it is employed in continuous bunstock manufacture. The problem can be alleviated by installing exhaust hoods and the like over those sections of the bunstock apparatus in which the major portion of volatilization of the isocyanate occurs. However, since the bunstock apparatus usually extends for considerable distances, of the order of 100 feet or more, it is generally impracticable to install venting means throughout the length of the conveyor to exhaust the diisocyanate fumes.
The above problem has become even more acute because of the rigid standards imposed by state and federal authorities on the permissible level of polyisocyanates which can be discharged into the atmosphere.
We have now found that the above problem can be solved in a relatively simple but elegant manner by the process which is described hereinafter.